There is known a machine tool that includes a plurality of spindles arranged parallel to each other and simultaneously machines a plurality of workpieces with the respective spindles. In such a machine tool, a displacement occurs in a spindle spacing (a pitch between the spindles) due to a temperature change (e.g., a temperature difference between day and night) inside a factory and heat generated by the rotation of the spindles.
Further, similarly in jigs which grasp workpieces, a displacement occurs in the spacing between the jigs due to a temperature change inside the factory and contact of chips heated by machining. It is known that a displacement amount in the jigs differs from a displacement amount in the spindles because the jigs are typically made of a material different from the material of the spindles.
When such thermal displacement is left as it is, the left thermal displacement has an extremely bad influence on the workpiece machining accuracy. Thus, there has been conventionally performed fine adjustment of the spindle spacing corresponding to the relative positions with respect to the jigs in order to increase the machining accuracy. Patent Literature 1 describes a machine tool provided with a device that performs such adjustment of the spindle spacing.
FIG. 11 is a diagram illustrating a principal part of the machine tool of Patent Literature 1. Spindles 401, 501 are supported movably in the front-rear direction (the direction perpendicular to the sheet) on two machining units 400, 500, respectively. These machining units 400, 500 are coupled by a clamping device (not illustrated) and integrally movable in the right-left direction by an X-axis moving mechanism 600 installed on a bed. When the spindle spacing between the spindles 401, 501 supported on the machining units 400, 500 is corrected, a clamping state of the clamping device which couples the machining units 400, 500 is released, and the position of each of the machining units 400, 500 is corrected by a spacing correction device 700 which is separately provided. The spacing correction device 700 includes a ball screw 702 which is driven by a motor 701.